The present invention relates to a method and apparatus for the ultrasonic characterization of polymer melts under processing conditions. More particularly, the invention is directed toward monitoring changes in viscoelastic properties, structure and composition of flowing polymer melts during processing.
The ever increasing need for high quality products puts strong pressure on improving process control systems. This is clearly illustrated by the large number of attempts to monitor polymer properties during processing. On-line or side-stream rheometers have been developed to monitor viscoelastic properties of flowing polymer melts and to use these properties to control the process. Apart from problems inherent to mechanical devices, there are also a number of disadvantages associated with this test method. For example, only part of the flowing polymer is tested and the sampled polymer stream may not be representative of the main stream. In addition, the sampled stream will have a different shear history than the main stream.
Ultrasounds have found numerous uses in very widely different fields of application, in particular to characterize polymers, both in the solid and molten states. For instance, the ultrasonic characterization of polymers under simulated processing conditions has been described in U.S. Pat. No. 4,754,645. According to this patent, a sample of a polymer is held in confinement between two axially aligned buffer rods having opposed parallel end surfaces spaced from one another to define a gap filled with the polymer sample, the polymer sample being acoustically coupled to the opposed end surfaces of the buffer rods. Ultrasonic waves are transmitted through one of the buffer rods in a direction toward the polymer sample for interaction therewith, and the polymer sample is subjected to controlled temperature or pressure variations over a predetermined period of time, the variation in temperature or pressure being effected via the buffer rods. Phase and amplitude variations of the ultrasonic waves having interacted with the polymer sample are continuously monitored as well as thickness variations of the polymer sample, over the predetermined period of time, to obtain data comprising phase, amplitude and thickness values measured as a function of temperature or pressure and time. The data is then processed to derive characteristic parameters providing both a thermodynamic and viscoelastic characterization of the polymer.
The properties of polymers during actual processing, on the other hand, are known to be highly dependent on flow conditions, i.e., flow velocity, shear rate, pressure and temperature. The apparatus described in aforementioned patent is capable of simulating only the temperature and pressure conditions normally encountered in actual processing, and is thus limited to measuring static properties of the polymers as opposed to flow properties during processing operations.
In U.S. Pat. No. 4,509,360, on the other hand, an acoustical technique has been proposed for the on-line measurement of particle dispersion in polymer melts. The technique is based on the use of a cylindrical quartz lens mechanically coupling a piezoelectric transducer to the polymer melt. A repetitive, broadband, ultrasonic wave is produced by the transducer and is sent into the lens. The ultrasonic wave meets a spherical dimple at the end of the lens which focuses a portion of it into a diffraction limited spot or interrogation zone within the polymer melt. Unmixed or agglomerated particles in the melt passing through this zone can be detected by measuring changes in the back scattered wave which is monitored by the transducer.
Since use is made of a focussing acoustic lens, the technique described in U.S. Pat. No. 4,509,360 is limited to measuring only properties at the surface or in a very restricted volume of the polymer melt, and thus does not account for the overall properties of the bulk material. The technique does not permit measuring or monitoring the viscoelastic behavior which is intimately associated with the bulk properties. The technique also relies on a special or particular geometry of the acoustic lens which perturbates the flow characteristics and therefore renders it unsuitable for use in actual processing devices.